Caption: Ostraceous psoriasis. Symmetrical patches of scaly psoriatic skin on a man's elbows. The tough lamellar scales resemble the outside of an oyster shell, hence the name ostraceous psoriasis. Psoriasis is a condition in which skin cells are produced much faster than normal. The body cannot shed the cells fast enough, resulting in plaques of scaly, thick skin, which are often itchy and may become infected. Psoriasis is triggered by many factors including emotional stress, some infections or certain drugs, although in many cases there is no obvious cause. Emollient creams, phototherapy and drug treatments can be used if the cause is not known.